This invention relates generally to an automatic power transmission for use in motor vehicles and more particularly to an add-on overdrive unit adapted to provide a fourth forward-driving speed ratio for an existing three speed automatic transmission.
The U.S. Pat. No. 4,095,487 issued June 20, 1980 to Cartwright et al discloses a three forward speed and reverse drive motor vehicle automatic transmission adapted for use with an engine which extends transversely on the vehicle. There have been numerous overdrive devices proposed for establishing a reduction gear ratio of less than one for automatic transmissions of the type disclosed in the Cartwright patent. The advantages of such overdrive units include less engine noise during high speed operation and improved fuel consumption. Examples of overdrive arrangements for automatic transmissions are found in U.S. Pat. Nos. 4,453,429; 4,455,890; and 4,567,788.